Introduction
Viscosity is a fluid's resistance to flow. Water has low viscosity; honey has high; glass (over geological time) technically flows. The SI unit is pascal-second (Pa·s); the older CGS unit poise is still common in chemistry and oil industry. Engine oil '10W-40' ratings come from a different system (SAE) entirely.
Why dynamic viscosity units exist and how they diverged
The pascal-second (Pa·s) is the SI unit — 1 Pa·s means shear stress of 1 Pa produces shear rate of 1 per second. Water at 20°C is about 0.001 Pa·s = 1 mPa·s = 1 centipoise (cP). The centipoise is convenient because water's viscosity is almost exactly 1 cP. Honey is ~10,000 cP. Glass at room temperature is ~10¹⁹ cP (effectively solid, despite the myth about medieval cathedral windows being thicker at the bottom).
Kinematic viscosity (m²/s or Stokes) is dynamic viscosity divided by density — how fast momentum diffuses. Engine oil 'SAE 40' is a kinematic viscosity range at 100°C (~12.5-16.3 cSt).
How to convert dynamic viscosity
cP to Pa·s: multiply by 0.001. Poise (P) to Pa·s: multiply by 0.1. Dynamic to kinematic: divide by density. Water at 20°C: dynamic 1 cP = 0.001 Pa·s; kinematic 1 cSt = 10⁻⁶ m²/s.
Units supported by this dynamic viscosity calculator
- Pa·s (Pascal-second)
- Poise (P)
- Centipoise (cP)
- Millipascal-second (mPa·s)
- Reyn
- lbf·s/ft²
- lbf·s/in²
Common dynamic viscosity conversion mistakes
- Dynamic vs kinematic viscosity. Dynamic (Pa·s, cP) measures force needed to shear. Kinematic (m²/s, cSt) = dynamic / density. Engine oil ratings typically kinematic; rheology papers typically dynamic.
- SAE engine oil grades. 10W-40 means low-temp performance like a 10 and 100°C performance like a 40. Not a direct viscosity number.
- Temperature dependence. Viscosity of most liquids drops rapidly with temperature. Honey at 10°C is 3-5× thicker than at 25°C.
- Shear-thinning fluids. Ketchup, paint, blood have different viscosity at different shear rates. A single 'viscosity' value doesn't capture the behavior.
Real-world dynamic viscosity examples
- Water (20°C): 1 cP = 0.001 Pa·s.
- Air (20°C): 0.0181 cP.
- Blood (37°C): 3-4 cP (shear-dependent).
- Milk: 3 cP.
- Olive oil: 84 cP at 20°C, 40 cP at 40°C.
- Honey: 10,000 cP at 20°C.
- Corn syrup: 50,000-100,000 cP.
- Peanut butter: 150,000-250,000 cP.
- Pitch (asphalt): 2 × 10¹¹ cP (the famous University of Queensland pitch-drop experiment).
Tips for accurate dynamic viscosity conversion
- For food science, cP is the practical unit — water is 1, everything else scales.
- For engine oil, SAE grades (5W-30, 10W-40) are easier than direct viscosity values.
- For lab work, always measure at a stated temperature — viscosity is strongly temperature-dependent.
Related: Density Converter · Flow Rate Converter · Pressure Converter.